As transmission capacities and speeds of trunk lines in a telecommunications network are increasing, fiber-optic communication has become a main transmission means on a modern information network. A photonic integrated circuit (PIC) chip is a core constituent part of an optical communications module and component. When the chip is connected to an optical fiber, polarization states of optical signals entering the PIC chip from the optical fiber are uncertain because a section of the optical fiber is not strictly circular and other factors such as stress are exerted. When these optical signals in the uncertain polarization state pass through the PIC chip directly, effective refractive indices are different in a transverse electric wave (TE) mode and a transverse magnetic wave (TM) mode in the PIC chip because of restrictions such as a birefringent effect or a waveguide size. Consequently, most components are sensitive to polarization, a polarization dependent loss (PDL) and polarization mode dispersion (PMD) are generated, and further a signal-to-noise ratio of a system is reduced. Currently, to ensure signal transmission quality and eliminate a polarization effect, a PIC chip needs to separately process optical signals in different polarization states. In this case, a polarization mode converter is an indispensable constituent element of a PIC chip.
In other approaches, a polarization mode is converted using two adjacent waveguides of a same height, and widths of the two adjacent waveguides remain unchanged in an optical signal transmission direction. The polarization mode converter has a small process tolerance and a high defect rate.